Metal-air batteries are electro-chemical reactors that produce electric current by oxidizing metal with oxygen. These reactors can have high energy densities and be relatively inexpensive to produce. Sizes can range from the small to power hearing aids or cameras to the large to power vehicles.
A mass of metal can form a porous anode that is saturated with an electrolyte. During discharge, oxygen reacts at a cathode to form hydroxyl ions that migrate into the metal/electrolyte paste to form a metal hydroxide, releasing electrons to travel to the cathode. The metal hydroxide decays into metal oxide and the resulting water returns to the electrolyte. The water and hydroxyls from the anode are recycled at the cathode, so the water is not consumed. The reverse process can also occur. During charge, electrons react with the metal oxide to reform the metal, releasing hydroxyl ions that migrate to the cathode. The hydroxyl ions are then oxidized to oxygen gas and water.